Scanning electrochemical microscopy (SECM) is a scanned probe microscopy technique which uses the electrochemical response of probe (typically an ultramicroelectrode (UME) with an active part of typically less than 25 μm) to provide information on the properties (topography or chemical activity) of a surface of interest (an interface, surface or phase) or for modification of a surface of interest (an interface, surface or phase). The SECM probe is immersed in a solution and used to detect chemical species (typically molecules or ions) which interact with the SECM probe or to generate chemical species (typically molecules or ions). Because the SECM probe detects or generates concentration and fluxes of the chemical species locally, the probe provides information on the properties of an interface, surface or phase at high spatial resolution. The SECM probe can be operated in amperometric, potentiometric or conductivity modes, amongst other possibilities. A wide variety of SECM probes have been described including solid metal electrodes, semiconducting electrodes, hybrid optical/electrode probes and liquid/liquid probes.
Conventional amperometric SECM (which forms the majority of applications) typically uses a metallic UME, although other probes are possible, operated in direct current (DC)-constant height (CH) mode. The UME is held at a potential to detect or electrolyze an analyte at a diffusion-limited rate and is positioned above the interface of interest.
The probe is then moved laterally, at a constant height, across the interface of interest and the electrochemical response, as the measured current, of the probe is recorded as a function of the tip position. This provides a current image which depends on both the sample topography (distance between the probe and the interface) and surface activity.
Despite the impact of SECM in interfacial science, a significant challenge concerns absolute probe positioning. Intermittent contact SECM (IC-SECM) provides a method to resolve the probe to substrate separation independently of the probe electrochemical signal. This is not only valuable for amperometric probes, but also potentiometric probes, which can be used more easily in SECM with this positioning approach. In IC-SECM the SECM probe, usually aligned perpendicular with respect to the surface of interest, is oscillated vertically with respect to the substrate of interest, with small amplitude compared to the probe tip size. When the probe approaches a surface of interest, and eventually comes into intermittent contact with the substrate, the vertical oscillation of the probe is damped, and this damping is detected and used to maintain the probe tip to substrate separation.
IC-SECM was described in patent application PCT/GB2011/050747, the contents of which are incorporated herein by reference, and it was also reported in “Intermittent Contact-Scanning Electrochemical Microscopy (IC-SECM): A New Approach for Tip Positioning and Simultaneous Imaging of Interfacial Topography and Activity” (McKelvey, K.; Edwards, M. A.; Unwin, P. R. Anal. Chem. 2010, 82, 6334-6337). IC-SECM has been used to image the flow of electroactive species through dentin (see “Quantitative Visualization of Molecular Transport through Porous Membranes: Enhanced Resolution and Contrast using Intermittent Contact-Scanning Electrochemical Microscopy” McKelvey, K.; Snowden, M. E.; Peruffo, M.; Unwin, P. R. Anal. Chem. 2011, 83, 6447-54.)